Temporal trends in 137Cs concentrations in the bark, sapwood, heartwood, and whole wood of four tree species in Japanese forests from 2011 to 2016

Abstract

To understand the changes in radiocesium (Cs) concentrations in stem woods after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, we investigated Cs concentrations in the bark, sapwood, heartwood, and whole wood of four major tree species at multiple sites with different levels of radiocesium deposition from the FDNPP accident since 2011 (since 2012 at some sites): Japanese cedar at four sites, hinoki cypress and Japanese konara oak at two sites, and Japanese red pine at one site. Our previous report onCs concentrations in bark and whole wood samples collected from 2011 to 2015 suggested that temporal variations were different among sites even within the same species. In the present study, we provided data on bark and whole wood samples in 2016 and separately measured Cs concentrations in sapwood and heartwood samples from 2011 to 2016; we further discussed temporal trends inCs concentrations in each part of tree stems, particularly those in Cs distributions between sapwood and heartwood, in relation to their species and site dependencies. Temporal trends in bark and whole wood samples collected from 2011 to 2016 were consistent with those reported in samples collected from 2011 to 2015. Temporal variations inCs concentrations in barks showed either a decreasing trend or no clear trend, implying that Cs deposition in barks is inhomogeneous and that decontamination is relatively slow in some cases. Temporal trends inCs concentrations in sapwood, heartwood, and whole wood were different among species and also among sites within the same species. Relatively common trends within the same species, which were increasing, were observed in cedar heartwood, and in oak sapwood and whole wood. On the other hand, the ratio of Cs concentration in heartwood to that in sapwood (fresh weight basis) was commonly increased to more than 2 in cedar, although distinct temporal trends were not found in the other species, for which the ratio was around 1 in cypress and pine and below 0.5 in oak, suggesting thatCs transfer from sapwood to heartwood shows species dependency. Consequently, the species dependency of Cs transfer within the tree appears easily, while that from the environment to the trees can be masked by various factors. Thus, prediction ofCs concentrations in stem wood should be carried out carefully as it still requires investigations at multiple sites with a larger sample size and an understanding of the species-specific Cs transfer mechanism.

New predictions of 137Cs dynamics in forests after the Fukushima nuclear accident

“…S2). The simulated flux converged more quickly for pine trees (3 years) than for oak trees (5-7 years), and, as seen by the continuously rising 137 Cs activity concentration in oak wood 3,15 , the uptake flux was greater than the downward flux in oak trees. The observed 137 Cs activity concentration in cedar and oak wood showed different trends, and the modelling well reproduced the overall trends ( Fig.…”

“…However, some potential limitations, which are sources of uncertainty, should be stated. Observations revealed that the 137 Cs activity concentration of sapwood and heartwood differed, particularly with that of cedar 15 . The prediction of 137 Cs activity concentration in wood may require explicitly dividing the sapwood and heartwood when the concentration in sapwood and heartwood differ considerably.…”

“…Data. For the simulation of temporal changes in the inventory data, we parameterized the model using the inventory data on the 137 Cs observed in the study site 3,5,15,38 in Otama village (37°34′N, 140°18′E) of Fukushima prefecture. There are three monitoring sites in the Otama site: 1) sugi cedar forest (Cryptomeria japonica: corresponding to evergreen needleleaf forest in the previous prediction), 2) konara oak (Quercus serrata) dominant mixed forest with red pine (Pinus densiflora) (corresponding to deciduous broadleaf forests in the previous study), and 3) red pine dominant mixed forest with konara oak.…”

“…For the calibration and the simulation of temporal changes in the 137 Cs activity concentration for cedar and oak wood, we used the observation data obtained in the Otama and Kawauchi sites 3,5,15,38 (Kawauchi site: 37°17′N, 140°48′E) in Fukushima prefecture. The activity concentrations were also normalized using the total initial inventory.…”

Effectiveness of landscape decontamination following the Fukushima nuclear accident: a review

“…Radiation dose rates were also measured among different types of workers taking part in professional activities in the village of Kawauchi, where the annual doses of foresters (range: 0.7-1.9 mSv yr −1 ) were not significantly higher than those of farmers (0.7-1.5), builders (0.6-1.5), office workers (0.5-1.5) and unemployed individuals (0.5-1.7). In contrast, decontamination workers (0.5-7.1) were found to have significantly higher dose rates (Orita et al, 2017).…”